The result: WikiLeaks is now stronger than ever, at least as measured by its ability to publish online.

Blocked from using one Internet host, WikiLeaks simply jumped to another. Meanwhile, the number of "mirror" Web sites - effectively clones of WikiLeaks' main contents pages - grew from a few dozen last week to 200 by Sunday. By early Wednesday, the number of such sites surpassed 1,000.

At the same time, WikiLeaks' supporters have apparently gone on the offensive, staging retaliatory attacks against Internet companies that have cut ties to the group amid fears they could be associated with it. On Wednesday, hackers briefly shut down access to the Web sites for MasterCard and Visa, both of which had announced they had stopped processing donations to WikiLeaks.

WikiLeaks' long-term survival depends on a number of unknowns, including the fate of its principal founder, Julian Assange, who is being held in Britain while awaiting possible extradition to Sweden related to sexual-assault allegations. But the Web site's resilience in the face of repeated setbacks has underscored a lesson already absorbed by more repressive governments that have tried to control the Internet: It is nearly impossible to do.

Experts, including some of the modern online world's chief architects, say the very design of the Web makes it difficult for WikiLeaks' opponents to shut it down for more than a few hours.

"The Internet is an extremely open system with very low barriers to access and use," said Vint Cerf, Google's vice president and the co-author of the TCP/IP system, the basic language of computer-to-computer communication over the Internet. "The ease of moving digital information around makes it very difficult to suppress once it is accessible."

Thus, despite the global uproar over the release of sensitive U.S. diplomatic cables, Assange's Web site remained defiantly intact Wednesday. Over the past week it has continued to publish a steady stream of leaked State Department documents with little visible evidence of injury from repeated, anonymous cyber-attacks or the multiple attempts to cut off its access to funding and Web resources.

By contrast, companies that have pulled the plug on WikiLeaks have suffered publicly, with cyber-attacks rendering their Web sites inaccessible or slow for hours at a time.

While a group of "hacktivists" targeted MasterCard and Visa - part of "Operation Payback," they called it - anonymous assailants have also in recent days attacked PayPal, which severed relations with WikiLeaks citing violations of its terms of service.

Web sites for Swedish prosecutors and a Swedish lawyer have also been hit, as has the banking arm of the Swiss postal service, which said it had frozen Assange's account, and even the Web site of former Alaska governor Sarah Palin.

WikiLeaks' seeming invulnerability is seen by experts as a demonstration of the power of new Web-based media to take on not only governments but also the traditional news media.

The group prides itself as an organization without a country - it has supporters worldwide but no central headquarters that would make it vulnerable to legal and political pressure. The organization's Internet infrastructure is spread over several continents, making it harder for outsiders to knock the site offline.

For those reasons, experts say, WikiLeaks remained relatively unscathed last week when the site's main domain name - wikileaks.org - was deactivated by its New Hampshire-based domain-name manager. Within days, WikiLeaks had signed up with more than a dozen other firms scattered across Europe, Canada and Asia.

WikiLeaks also simultaneously posted an appeal to its supporters, asking them to voluntarily host "mirror" sites. Hundreds of individual Web servers signed up, from countries around the world.

Similarly, WikiLeaks found new avenues for processing donations after PayPal and MasterCard announced they would no longer service payments for the group. The effect on the organization's financial health is not yet clear.

Inevitably, efforts to restrict sites such as WikiLeaks through financial and regulatory pressures will fall short, for the same reasons that government regulators have been unable to shut down purveyors of Internet spam, or various Web-based criminal enterprises, said Paul Vixie, president of Internet Systems Consortium, a nonprofit Internet infrastructure company in Redwood City, Calif.

"Something that's illegal in some countries but not others is very hard to keep off the Net, even though there's been some success in keeping it out of the countries where it's illegal," Vixie said. "If WikiLeaks is willing to spend as much money as e-criminals . . . they could probably remain online indefinitely."

The isolation of WikiLeaks has prompted cries of censorship and government interference.

"I can use my credit card to send money to the Ku Klux Klan, to antiabortion fanatics, or to anti-homosexual bigots, but I can't use it to send money to WikiLeaks," said Jeff Jarvis, a new-media critic and director of the interactive journalism program at the City University of New York's Graduate School of Journalism. "The New York Times published the same documents. Should we tell Visa and MasterCard to stop payments to the Times?"

It is ironic, Jarvis said, that the U.S. protests against Assange's campaign of leaks come weeks after Secretary of State Hillary Rodham Clinton criticized Chinese efforts to restrict freedom of the Internet. While Western governments are used to seeing secrets leaked through traditional media, they are struggling to adjust to a new era in which raw data can be easily and rapidly disseminated around the world.

"There is an information war, and it's about control," he said. "The choice is to either live in a transparent world or shut down the Internet."